Surgical options to treat tissue surplus are generally successful in achieving the desired goals of reduced tissue mass and restoration of normal tissue geometry. Procedures of this nature include ostectomy, mastectomy, partial and complete hepatectomy. However, when tissue deficiencies are present and there is a need or desire to increase tissue mass, therapeutic options become more involved, and less certain in outcome. Options to increase tissue mass include the use of autografts, allografts, xenografts and alloplastic materials. Autografts involve the transfer of tissue from one part of the patient to another (either as a vascularized graft or as a non-vascularized graft). The main drawbacks of autograft therapies are; the limited amount of tissue that is available for transfer, donor site morbidity and, in some cases, the complete lack of available or appropriate donor sites. In addition, in the case of non-vascularized bone grafts, resorption of the transferred tissue can result in decreased tissue mass and inadequate function and/or aesthetic outcome.
When autograft tissue is not available, allografts may often be used. Allografting involves the transfer of tissue between two individuals of the same species. Such procedures are not without problems however. Associated problems with this technique may include lack of donors, immunological response of the host, the need for immunosuppression to prevent immune rejection of the transferred tissue, revitalization of the grafted material by the host, and the possibility of disease transfer from donor tissue to the recipient. Xenograft therapies (transplantations from one species to another) circumvents the tissue supply problems associated with allografts, however the problem of xenograft immune rejection has yet to be solved. Immuno-isolation techniques involving encapsulation of xenograft cells show promise in some applications, especially those related to metabolic tissues, but have yet to reach clinical efficacy.
Much attention has been paid to the possibilities of generating or regenerating tissues. In the case of tissues which have some potential for self-regeneration (such as bone, cartilage and nerve), porous matrices, which are usually biodegradable, have been used to direct tissue formation. However, these regenerative processes are dependent on, and limited by, both device design and the regenerative potential inherent in the biological processes of the individual. This dependence may affect rate of formation, quantity and architecture of the resulting tissue. In the case of bone, porous materials, such as coralline hydroxyapatite and certain preparations of allograft bone, have been used as scaffolds to facilitate tissue growth into bony defects. This approach has been successful in instances associated with small defects but lacks the desired predictability of outcome in many clinically relevant large defects. Interaction of the host cells, e.g. the so called foreign body reaction, with the porous matrix also may limit the rate, quantity and architecture of the tissue formed within the device.
Recent research has also focused on the use of bioactive molecules or transplanted cells that have the potential to stimulate tissue formation. The local administration of cells or bioactive molecules alone is insufficient and does not result in predictable regeneration of tissue masses (Bessho 1996). Research efforts have therefore focused on the use of carriers to deliver bioactive molecules or act as scaffolds for transplanted cells. In addition the carriers act as scaffolds to direct cell growth and tissue formation. Such carriers usually take the form of space filling devices, such as three-dimensional porous networks, gels, microspheres or granular materials. Membranes which create and maintain a space for tissue regeneration have also been used as carriers for bioactive molecules.
Space filling devices have been used extensively in the field of bone regeneration to act as carriers for bioactive molecules known to stimulate bone formation (Wolfe and Cook, 1994). The materials used to fill a space where bone formation is required vary widely in their structural geometry and mechanical properties and include porous hydroxyapatite, allograft bone, collagen sponge and degradable polymer foams, scaffolds (Brekke, U.S. Pat. No. 5,683,459) and microspheres. These various approaches to bone regeneration each suffer one or more drawbacks.
For example, relatively strong and rigid materials, such as porous coralline hydroxyapatite, can withstand the forces created by surrounding soft tissue, wound contracture and local load induced stresses. These materials have the capacity to resist collapse of the geometric form defined by the material and thus maintain the original shape of the space occupied by the porous matrix. However, these materials also take up a significant proportion of the space which could otherwise be occupied by newly formed bone. As a consequence, such materials interfere with bone formation and also result in a potentially detrimental interface between bone and the biomaterial. The presence of a biomaterial interposed within bone can interfere with normal bone remodeling processes and can ultimately result in bone resorption or stress fractures (Spector 1991). In addition, with scaffolds or carriers constructed from synthetic degradable polymers, such as poly(a-hydroxyesters), a relatively large volume of material resulting in lower porosity, is required to produce a structure with sufficient strength. Consequently, there is less space available for bone formation and significantly greater quantities of degradation products. These degradation products can interfere with bone formation and can also result in bone resorption (Bostman 1992).
Many of these problems may be circumvented through the use of carriers such as collagen sponges which do not appear to significantly interfere with tissue formation or remodeling even during the degradation phase of the material. For example, Ksander et al. (U.S. Pat. No. 4,950,483), Chu et al. (U.S. Pat. No. 5,024,841) and Chu et al. (U.S. Pat. No. 5,219,576) describe a space filling collagen sponge with pores greater than 35 microns which may be used in conjunction with bioactive agents to promote wound healing. However, collagen sponges, especially those with suitable degradation time frames, are generally not able to withstand the above-listed in vivo stresses and consequently are unable to maintain the size and shape of the filled space. As a result, the newly created tissue assumes an ill-defined geometry which is not the same as the original shape of the sponge and which is often not of adequate or optimal functional or therapeutic benefit. Such an outcome is reported by Oppermann et al. (U.S. Pat. No. 5,354,557) who describes the use of a collagen sponge combined with osteogenic proteins for bone regeneration. Wolfe and Cook (1994) have also recognized the difficulty of controlling the geometry of bone formed using osteogenic proteins and state that "the osteoinductive effect of the protein can be difficult to confine to a limited anatomic area, especially using semi-solid carrier vehicles." This same phenomenon can be seen with degradable synthetic polymer structures that, although they can be designed with appropriate mechanical stresses at the time of implantation, gradually lose their ability to withstand in vivo stresses and collapse at some point during degradation.
Kuboki et al. (1995) used bioactive molecules in conjunction with a flat, space-filling, unwoven glass fibril membrane to study bone formation. In this case, the majority of the tissue formed was cartilage that was located within the microstructure of the membrane. The desired bone tissue was therefore not formed and the cartilage tissue generated was associated with a non-degradable material that is likely to adversely influence the desirable properties of the natural tissue. Chu et al. (U.S. Pat. No. 5,219,576) describe a space filling collagen matrix with a thickness of 1-20 mm and a pore size of at least 30 mm in diameter. The matrix is intended for use in skin/dermal wound healing and tissue regeneration and can be used in conjunction with bioactive molecules. The teachings of Chu et al. do not address the necessity of controlling the configuration of the tissue generated by the articles and methods of the invention.
An improvement of these tissue regeneration methods is found in the field of guided tissue regeneration. Guided tissue regeneration is a therapeutic approach aimed at regenerating periodontal tissues and bone, particularly of the jaw. It is based on the concept of selective tissue exclusion and attempts to optimize the natural regenerative potential of the patient's tissues. Exclusion of undesirable fibrous tissue from the regenerative space is achieved through the use of a membrane that acts as a passive physical barrier which is substantially impermeable to cells and tissues. In addition, the membrane maintains the regenerative space until such time as the tissue is regenerated. This technology has done much to advance the field of alveolar bone and periodontal tissue regenerative therapies, however, the widespread application of this technology has not been fully realized due to issues of predictability in the most clinically challenging situations.
Scantlebury et al. (U.S. Pat. No. 5,032,445) described the potential for a combination of tissue excluding guided tissue regeneration membranes and bioactive molecules. This concept was reiterated by Golgolewski (U.S. Pat. No. 5,676,699 and EP 0 475 077) who described a degradable, microporous bone regeneration membrane which may be used in conjunction with various bioactive molecules. Again, the membrane was used as a "tissue separator which promotes and protects osseous regeneration." According to Golgolewski, the essential function of the micropores is that they are "permeable for nutritional fluids." This statement is consistent with the most preferred pore diameter range of 0.1 to 5.0 mm. Similar concepts have been reported by Sottosanti (U.S. Pat. No. 5,569,308), Dunn et al. (U.S. Pat. No. 5,077,049), Jernberg (U.S. Pat. No. 5,059,123 and U.S. Pat. No. 5,197,882) and Hehli (U.S. Pat. No. 5,383,931). Saffran (U.S. Pat. No. 5,446,262) for example, describes a two layered, tissue excluding membrane for the directional delivery of bioactive molecules for tissue repair. A bi-layered tissue excluding membrane in combination with a bioactive molecule is also described by Aebischer et al. (U.S. Pat. No. 5,011,486) for nerve regeneration.
The combination of bioactive molecules with guided tissue regeneration membranes has also been studied in small, experimental defects in rabbit long bones by Zellin and Linde (1997). In this model the combination of a bioactive molecule with a cell and tissue excluding membrane was successful in regenerating the bone defect. However, studies by Cochran et al. (1997) show that bone formation in the mandible is impeded by the use of a cell and tissue excluding membrane in combination with a bioactive molecule. In addition, studies by Hedner and Linde (1995) found that the combination of a cell and tissue excluding membrane and a bioactive molecule was less effective in stimulating bone healing in mandibular defects than the bioactive molecule alone. Predictable bone regeneration using a bioactive molecule and a membrane which excludes cells and tissue from the regenerative space has yet to be shown.
Khouri et al. (1991) describes the use of an osteogenic protein with a non-porous silicone rubber mold filled with a vascularized muscle flap and demineralized bone matrix. The muscle flap was transformed into bone which matched the shape of the mold. One of the disadvantages of this technique is that it requires the use of living autogenous tissue, namely a muscle flap, which requires surgically injuring existing living tissue. In addition, vascularized muscle tissue may not be available for transplantation in specific areas, such as the oral cavity, especially without significantly affecting other functions of the patient.
Boyne (1996) used rhBMP-2 in conjunction with titanium orthopaedic plate mesh (TiMesh) to regenerate bone in a monkey mandible model. This mesh has a hole size of 2.2 mm and was used only as a mechanical support to stabilize the bone ends. The mesh device used does not define the configuration of a space having the size and shape desired for the bone to be generated. Some tissue sections from Boyne show extensive bone formation beyond the boundary of the mesh.
Thompson et al. (WO 89/07944) describe a device and methods for stimulating and directing the formation of vascular tissue through the use of a biocompatible support (geometry and structure undefined) in conjunction with a bioactive agent. The reference teaches the creation of an amorphous vascular bed as opposed to a tissue of specific architecture or geometry.
The use of membranes which are not tissue excluding has been studied by Pineda et al. (1996) for long bone regeneration. This study showed that the principles and mechanisms of guided bone regeneration do not operate when membranes are utilized with larger pore sizes that do not result in cell and tissue exclusion. Consequently, significantly less bone regeneration results with large pore size membranes which do not exclude cells and tissue. Haris (U.S. Pat. No. 4,787,906) describes a similar system for alveolar ridge augmentation which utilizes inert particles contained within a porous tube designed to allow tissue through growth. The teachings of Haris specify fibrovascular invasion into the tube, but this invention does not teach the use of bone inductive agents.
Tepic (U.S. Pat. No. 5,211,664 and EP 0 551 611) teaches the use of a structure for long bone regeneration which comprises two concentric, parallel, tubular shells connected to each other by struts. One or both of the shells may be provided with interconnected micropores and therefore, according to the author, "diffusion alone is sufficient to maintain grafted metabolism in the critical phase before revascularization takes place." The author further states that the shells of the concentric tubular structure may also have "larger openings in the range of 0.1 to 2.0 mm" which "allow for vascular ingrowth from surrounding tissue." However, the author espouses adherence to the above-summarized teachings of guided tissue regeneration in which a membrane or sheet structure is used to substantially exclude soft tissue and soft tissue ingrowth from the space created by the membrane. Indeed, the most preferred embodiments are consistent with the teachings of guided tissue regeneration and stipulate the use of a concentric membrane structure with pore diameters in the range 0.1 to 5.0 mm. The structure is also intended to serve as a container for bone grafts or various bioactive agents.
The use of macroporous membranes in conjunction with autograft bone for long bone regeneration has been studied by Gerber and Golgolewski (1996) and later by Gugala and Golgolewski (1997). In these studies, a long bone defect was filled with autograft bone and covered with a macroporous membrane. Although these studies showed that it is possible to regenerate bone in this manner, this approach also resulted in massive resorption of the graft. In order to ameliorate the undesirable resorption outcome, two concentric macroporous membranes were required; one inserted into the medullary canal and the other placed on the periphery of the defect with the annular space filled with autograft bone. It is clear that long bone defects do not heal appropriately in the long term using a combination of autograft bone and a single macroporous membrane. Furthermore, in order to appropriately utilize the regenerative potential of autograft bone in long bone defects a highly specific device design involving concentric macroporous membranes is required. An additional drawback of this methodology is that it requires the use of autologous tissue in quantities at least sufficient to fill the defect.
Lemperle et al. (1996) has studied the use of a titanium mesh as a containment system for autograft bone. These studies have shown that with this system, it is possible to regenerate only as much bone over a 4 month period as is regenerated using a titanium mesh placed over an empty defect. Holmes (1997) has also postulated the use of a resorbable macroporous sheet as a containment system for bone grafts and Patyk (DE 91 15 341) describes a similar system for bone substitute materials. However, as the studies of Gugala and Golgolewski show, the use of a single macroporous sheet in conjunction with autograft bone does not result in a satisfactory long term healing response at least for long bone applications. In addition, this approach also requires the use of autologous tissue to fill the defect.
Teixeira and Urist (1997) describe the use of macroporous membrane, with pores 0.5 mm in diameter, in conjunction with a mixture of bovine derived bone morphogenetic protein and associated non-collagenous bone matrix protein for long bone regeneration.
Lemperle (WO 98/07384) describes a macroporous membrane structure for tissue reconstruction. The reference teaches that regeneration of bone and other tissues may be achieved solely through the use of a macroporous membrane which prevents prolapse of the surrounding soft tissue and allows ingrowth of blood vessels and connective tissue. However, as the long bone regeneration studies of Pineda et al. (1996) show, the principles and mechanisms of guided bone regeneration do not operate when membranes are utilized with larger pore sizes that do not result in cell and tissue exclusion. Significantly less bone regeneration results with large pore size membranes which do not exclude cells and tissue.
Although the Lemperle reference focuses on the merits of the macroporous membrane structure in tissue and bone regeneration, mention is also made of the possibility of impregnating the membrane with substances for promoting the regeneration of different tissues such as bone and blood vessels. Although delivery of a bioactive substance from a membrane may appear to be an attractive proposition, there are significant biological and technical shortcomings of this approach. It is technically difficult to achieve delivery of therapeutically effective quantities of an appropriate bioactive molecule from a membrane. For example, it is not always possible to load sufficient quantities of a bioactive agent onto a membrane by relying on simple adsorption of the molecules onto the membrane surface, especially if the membrane is constructed from a hydrophobic material. Materials which are hydrophobic in nature are most often used to construct membranes since they have superior mechanical properties over hydrophilic materials such as collagen.
Even if sufficient loading of the membrane can be achieved, the next obstacle is providing an appropriate delivery profile. In general, bioactive agents must be made biologically available to the host for a period of several days in order to stimulate significant tissue formation. This is why other tissue generation approaches use a controlled or sustained release device to deliver the bioactive agent over a sufficient period of time (Bessho, 1996). It is not generally possible to achieve such a sustained release profile via simple adsorption and desorption of the bioactive molecule from a membrane, however. An adsorption-desorption mechanism results in a so-called "burst effect" in which most of the bioactive substance is released in the first one or two days after implantation.
In addition to the technical difficulties involved in achieving appropriate delivery of a bioactive agent from a membrane, it has not been shown that delivery of a bioactive molecule from a macroporous and tissue penetrable membrane surrounding the periphery of a tissue defect is efficacious in regenerating a desired tissue. The presence of macropores in the membrane allows diffusion of the bioactive molecule both inward toward the center of the defect and outward toward the surrounding soft tissue. This has two distinct disadvantages. First, the presence of the bioactive molecule in high concentrations near the outer surface of the membrane would be likely to cause the desired tissue to form on the outside of the membrane thus resulting in a lack of control of the regenerated tissue geometry. Second, diffusion of the bioactive molecule inward toward the center of the defect would result in an adverse concentration gradient for the migration of cells, such as mesenchymal stem cells, from the tissue surrounding the defect. The migration of cells in response to a local concentration gradient is known as chemotaxis and is normally associated with cellular migration toward the highest concentration of a chemotactic substance. In the case of membrane delivered bioactive molecules, the lowest concentration would be at the center of the defect and the highest concentration at the surface of the membrane. Mesenchymal stem cells would therefore be expected to migrate toward the membrane and not toward the center of the defect where they are needed to facilitate tissue regeneration.
In summary, studies have been performed which utilize non-macroporous, tissue excluding membranes in conjunction with bioactive agents (Cochran et al. (1997) and Hedner and Linde (1995)). In each case, a bioactive agent delivered in an appropriate manner from a carrier material filling the space under a tissue excluding membrane did not achieve the desired bone regeneration result. In addition, desired bone regeneration is not attained when a macroporous membrane, which does not result in cell and tissue exclusion, is used alone (Pineda et al. (1996)). Although impregnating a macroporous membrane with bioactive molecules (Lemperle (WO 98/07384)) may result in bone formation, the regenerated tissue is unlikely to conform to the desired configuration defined by the geometry of the membrane.
Currently there is a need to predictably generate desired living tissue within the body of an individual and to control the configuration of the tissue generated without using autologous tissue. It would be highly desirable, therefore, to have a combination of a tissue penetrable device with a tissue stimulating molecular substance delivered from the space established by the device which results in predictable generation of desired living tissue while being able to control the configuration of the living tissue generated.